The invention relates to rectifying circuits that comprise a bridge connection with RC protected thyristors, a direct voltage circuit with capacitance and a charging circuit for charging the capacitance of the direct voltage circuit.
Rectifying circuits are an essential part of not only actual rectifiers, but also many other electrotechnical devices, such as frequency converters. Rectifying circuits often employ thyristors. The reverse-recovery currents generated in thyristors may cause high overvoltage, because electric circuits always contain inductances. For this reason RC protection is commonly used for protecting thyristors.
Rectifying circuits comprising a bridge connection with RC protected thyristors, a direct voltage circuit with capacitance and a charging circuit for charging the capacitance of the direct voltage circuit are conventionally implemented by connecting a series-connected capacitor and resistor in parallel with each thyristor in question, the charging circuit comprising a diode and a charging resistor connected in series.
The above arrangement involves many problems. Due to the high number of components, the costs arising from the components and their assembly are high. Moreover, the resistors used in RC protection are usually arranged on a printed board containing, in addition to the resistors, at least RC protection capacitors and thus the resistors not only take up part of the printed board surface, but also complicate the cooling of the control card space, for example, thereby shortening the life cycle of the cards and making their positioning problematic. Further, because the RC protection resistors' power handling capacity is poor, it is difficult to increase the capacitances in situations where it would be required due to issues related to EMC or earth fault tolerance, for example.